The invention relates generally to ink cartridges for ink jet printing systems, and more particularly to an improved cartridge having a high ink storage capacity that also prevents the spillage of ink.
Thermal ink jet printers typically have a printhead mounted on a carriage which traverses back and forth across the width of a movable recording medium such as paper. The printhead generally includes a array of nozzles that confront the recording medium. Each nozzle is located at one end of an ink-filled channel, the other end of which is connected to an ink supply reservoir. As the ink in the vicinity of the nozzles is used, it is replaced by ink in the reservoir. Small resistors in the channels are individually addressed by current pulses that represent digitized information or video signals. The thermal energy from the resistors causes droplets to be expelled from the nozzle and propelled onto the recording medium, where each droplet prints a picture element or pixel.
It is important that the ink at the nozzle be maintained at a negative pressure (sub-atmospheric pressure) so that the ink is prevented from dripping onto the recording medium unless a droplet is expelled by thermal energy. A negative pressure also advantageously ensures that the size of the ink droplets ejected from the nozzle remain constant as ink is depleted from the reservoir. The negative pressure is usually in the range of -0.5 to -2.0 inches.
A known, very simple method of supplying the ink at a negative pressure is shown in FIG. 1. The ink in container 6 has a maximum ink supply level 2 that is 0.5 inches below the printhead 1. The bottom of the container 6 is 2.0 inches below the printhead. The ink is drawn up the ink supply tube 3 by capillary action of print head nozzles. As long as the container 6 has an aperture 4 exposed to atmospheric pressure, this configuration will supply ink to the printhead 1 through the ink supply tube 3 at a negative pressure of 0.5 to 2.0 inches. The disadvantages of this configuration are that if the cartridge is not held upright the ink will spill out of the nozzles, and that the volume of ink available to the printhead is limited by the available volume in the machine, below the printhead nozzles.
Another known method of supplying ink at a negative pressure is shown in FIG. 2. In this configuration, the chamber 6 is filled with a foam in which the ink is suspended by capillary action. The foam is generally a partially saturated, reticulated urethane foam. The absorption of the ink by the foam maintains the ink at a negative pressure at the printhead 1. The value of the negative pressure is determined by a number of factors, including the properties of the foam selected, the surface tension of the ink, the height of the foam with respect to the printhead 1, and most importantly, the saturation of the foam. If the foam is filled with ink to 100% of its capacity, the ink will behave as if the foam were not present and thus there will be no negative pressure. An inherent advantage of a partially saturated foaming design is that because the ink is absorbed by the foam, ink will not spill regardless of the orientation of the cartridge. This is particularly advantageous during the shipping of the cartridge. However, a significant disadvantage of this design is its volume inefficiency; the cartridge needs a relatively large volume to supply a given quantity of ink. For example, a cartridge of this type manufactured by the Hewlett-Packard Corporation has a volume of 45 cc which can supply only 22 cc of usable ink. Thus, this cartridge has an efficiency of less than 50%.
Given the problems associated with these ink delivery systems, there is a need for an ink jet cartridge that has an improved volume efficiency while additionally minimizing the likelihood of spillage.